Polyoxometalates (POMs) are a unique class of inorganic metal-oxygen clusters. They consist of a polyhedral cage structure or framework bearing a negative charge which is balanced by cations that are external to the cage, and may also contain centrally located heteroatom(s) surrounded by the cage framework. Generally, suitable heteroatoms include Group IIIa-VIa elements such as phosphorus, antimony, silicon and boron. The framework of polyoxometalates comprises a plurality of metal atoms (addenda), which can be the same or different, bonded to oxygen atoms. Due to appropriate cation radius and good π-electron acceptor properties, the framework metal is substantially limited to a few elements including tungsten, molybdenum, vanadium, niobium and tantalum.
In the past, there have been increasing efforts towards the modification of polyoxoanions with various organic and/or transition metal complex moieties with the aim of generating new catalyst systems as well as functional materials with interesting optical, electronic and magnetic properties. In particular, transition metal substituted polyoxometalates (TMSPs) have attracted continuously growing attention as they can be rationally modified on the molecular level including size, shape, charge density, acidity, redox states, stability, solubility etc.
For example, Neumann et al. describe the preparation of ruthenium-substituted “sandwich” type polyoxometalate [WZnRu2(OH)(H2O)(ZnW9O34)2]11− as well as its ability to catalyze the oxidation of alkanes and alkenes using hydrogen peroxide and molecular oxygen as the oxygen donor (see: Angew. Chem. Int. Ed. Engl. 1995, 34, 1587; Inorg. Chem. 1995, 34, 5753; and J. Am. Chem. Soc. 1998, 120, 11969). Moreover, Pope et al., J. Am. Chem. Soc. 1992, 114, 2932, disclose the synthesis of the cesium salt of [PW11O39Ru(H2O)]4+ and characterize its oxygen atom transfer reactivity. In all these syntheses, RuCl3.nH2O or [Ru(H2O)6](C7H7SO3)2 are used as ruthenium sources.
Nomiya et al., J. Chem. Soc., Dalton Trans. 2001, 1506, discuss the difficulty of making pure Ru-containing POMs and the nonreproducibility of some reported Ru-substituted polyanions.
Recently, the dimethyl sulfoxide (dmso) complex cisRu(dmso)4Cl2 has become a popular ruthenium(II) source for the synthesis of Ru-substituted POMs. For example, Kortz et al. disclose the preparation and structural characterization of [HW9O33Ru2(dmso)6]7−, [Ru(dmso)3(H2O)XW11O39]6− (X=Ge, Si) and [HXW7O28Ru(dmso)3]6− (X=P, As) (see: Chem. Commun. 2004, 1420; J. Chem. Soc., Dalton Trans. 2004, 3184; and Chem. Commun. 2005, 3962).
However, up to now these RuII, (dmso)3-based anions have not turned out to be very useful for homogeneous or heterogeneous catalytic applications.
Therefore, it is an object of the present invention to provide a ruthenium-substituted polyoxometalate which is useful as catalyst in homogeneous and heterogeneous oxidation reactions of organic substrates. Furthermore, such a Ru-substituted POM should be easy and reproducible to prepare.
Other references of interest include:
Neumann et al., JOC, Vol. 68, No. 5, 2003; Finke et al. Inorg Chem. 2005; Neumann et al., Inorg. Chem. 2003 42(10), 3331-3339; WO 2003/028881; Xu et al., Fudan Xuebao, Ziran Kexueban 2001, 40(4), 424-428; Xu et al., Gaodeng Xuexiao Huaxue Xuebao 2001, 22(4), 520-523; U.S. Pat. No. 5,990,348; Tojima et al., JP 11199594-Mitsubishi; Rong, Diss. Abstr. Int. B 1993, 53(10), 5197; J. Org. Chem. 2003, 68, 1721-1728); Cavani et al. Topics in Catalysis 2003, 23, 141-152); and Kamat et al. Science 2003, 300, 964-966).